Permanent magnet machines or motors are commonly used in the industry to convert electrical or electromagnetic energy into rotational torque for a variety of different applications, including machine tools, traction motors, industrial work machines, stationary drive machines, mobile work vehicles, hybrid electric vehicles, and the like. One type of permanent magnet machine with growing interest in the industry is the interior permanent magnet (IPM) machine. Because of its relatively consistent power over a broad speed range and its enclosed-magnet design, IPM motors have shown to be ideal for many applications, especially for traction motors, machine tools, and the like.
As with typical induction motors, the IPM motor provides a multi-phase stator and a rotor disposed within the stator. The typical IPM motor is controlled by switching circuitry, or the like, which sources phase current to the different phases of the stator in succession, which in turn creates a changing electromagnetic field within the stator. Rotational torque is generated at the output shaft of the IPM motor as the permanent magnets disposed within the rotor attempt to align themselves according to the changing electromagnetic fields.
While different schemes can be used to drive an IPM motor, the typical drive scheme sources current to each phase of the stator in reference to feedback provided by a rotor speed sensor. Moreover, the rotor speed sensor serves to detect the rotational speed of the rotor relative to the stator, and provide a check and balance for the drive system as it operates the IPM motor. Although the rotor speed sensor can sufficiently monitor rotor speed, it is unable to track the absolute position of the rotor. Accordingly, typical drive schemes also incorporate means for determining the initial rotor position as a critical first step.
To determine the initial rotor position of an IPM machine, many conventional systems measure the voltage exhibited by the stator immediately after the rotor finishes spinning and comes to a rest. More specifically, the last point of zero-crossing of the rotor is assessed based on the stator voltage, and the associated angular position of the rotor is identified in relation to at least one phase of the stator. However, such initial position assessments are often inaccurate, and the inaccuracies are further compounded by the rotor speed sensor as it has no interim means to correct or compensate for such inaccuracies. The present disclosure is directed at addressing one or more of the deficiencies set forth above.